Commercial television receivers and set-top boxes supporting various digital television standards require tuner circuitry capable of processing analog terrestrial, digital terrestrial, and cable broadcasts with low cost and small size. Such tuner circuitry may be configured to handle signals with broadband input frequencies ranging from 54 MHz to 880 MHz, and thus require a wideband LNA with sufficiently high linearity and a low noise figure well below 3 dB in order to obtain high sensitivity.
Narrow-band receivers, such as Code-Division Multiple Access (CDMA), Global System for Mobile Communications (GSM), short-range wireless receivers, and terrestrial digital multimedia broadcasting (T-DMB) receivers, are typically configured to tune to channels where the ratio of maximum frequency to minimum frequency is less than two. Unlike such narrow-band receivers, in a hybrid television tuner, the second and third-order input-referred intercept points (IIP2 and IIP3) of the LNA are important. The IIP2 and IIP3 of the LNA are important because, in the frequency band of reception (54 MHz to 880 MHz), there can be an interfering channel with a frequency that is half or a third of the desired frequency channel, which interfering channel, through second order or third order distortion, respectively, can land at the frequency of interest and corrupt the picture quality. Such second or third order distortion products are called HD2 (harmonic distortion 2) and HD3 (harmonic distortion 3), respectively.
Furthermore, in a cable TV environment, there can be several channels distributed over the frequency band of reception. These channels can inter-modulate amongst themselves through second order nonlinearities and corrupt the desired picture quality through composite second order distortion (CSO). Furthermore, in the presence of third order nonlinearities in a cable TV tuner, the TV channels can inter-modulate to produce a composite triple beat (CTB) which can further affect the desired picture quality. Hence, it is important that an LNA in a TV tuner has high IIP2 and IIP3. Therefore, since an LNA with a single-ended input and single-ended output typically has poor IIP2 performance, many tuners adopt a balun together with a fully differential LNA or a single-ended-to-differential amplifier as the first stage of the receiver circuit.
While fully differential LNAs can provide acceptable IIP2 performance over a wide range of frequencies, one of its main drawbacks is that passive transformers are typically required to convert the single-ended signal into a differential signal. Such transformers can be bulky; therefore, such transformers are generally implemented off-chip. Further, low-loss, external, passive transformers are typically expensive, adding to the overall cost of the system. Furthermore, the loss in these external transformers directly affects the Noise Figure (NF) of the TV tuner, which is one of the most important performance metrics of a TV tuner because the NF determines how small of a broadcast TV signal can be received at the antenna input.
A single-ended-to-differential amplifier represents a good configuration for integration and can provide moderate IIP2 performance. However, conventional single-ended-to-differential amplifiers typically have a relatively high NF. Unfortunately, the high NF degrades sensitivity of the tuner circuitry, making it difficult to use an LNA on the first stage in a wideband tuner.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items.